1. Field of the Invention
The present invention relates to a method for drying a honeycomb formed body using a microwave.
2. Description of Background Art
A honeycomb structural body is widely used for a catalyst carrier and various filters, and recently has also attracted attention as a diesel particulate filter (DPF) for capturing particles discharged from a diesel engine.
In many cases, such a honeycomb structural body contains ceramics as major components. This type of honeycomb structural body is commonly manufactured by producing a kneaded compound (a clay) by adding water and various additives to a ceramic raw material, extruding the kneaded compound (the clay) into a formed body in the form of a honeycomb (a honeycomb formed body), and drying and sintering the honeycomb formed body.
A well known method of drying the honeycomb formed body is a dielectric drying method using high frequency energy generated by passing a current between electrodes installed in the upper part and lower part of the honeycomb formed body. A hot air drying method in which a hot wind produced by a gas burner or the like is used is also well known. In recent years, a drying method using microwaves is used in place of or in combination with these drying techniques due to the advantages of a high drying speed and the like (for example, Japanese Patent Application Laid-open No. 2002-283329).
Conventionally, honeycomb formed bodies have been dried by a microwave drying method by providing a drying furnace with a uniform electric field distribution and placing the honeycomb formed bodies in the furnace. As the method for providing a uniform electric field distribution, a method of adjusting the form and location of the antenna which radiates microwaves, a method of using a stirrer fan, and the like have been used.
However, these conventional microwave drying methods could not uniformly dry the entire honeycomb formed bodies because of slow drying speeds at the upper and lower ends and the perimeter of the honeycomb formed bodies. Since a honeycomb formed body shrinks due to vaporization of water, the drying speed difference (difference in the amount of water) inside the formed body causes cells in the honeycomb structural body to deform. The thickness of the ribs (partitions dividing cells) of honeycomb structural bodys have been reduced in recent years. The smaller the thickness of the rib, the easier the cell is deformed. For this reason, the importance of ensuring a uniform drying speed has increased.
The effects of heat dissipation from the honeycomb formed bodies, heat release, transpiring latent heat, and the like during the drying process are considered to cause the difference in the drying speed. In an effort to ensure a uniform drying speed of honeycomb formed bodies by microwave, a method of decreasing heat dissipation by increasing the temperature in the drying furnace to about 80° C., for example, by introducing vapor or hot air into the drying furnace, a method of decreasing the effect of heat dissipation by increasing the microwave output, and the like have been proposed. Although deformation can be suppressed to a certain extent using these methods, a sufficient decrease in the drying speed to provide a satisfactory difference in results is difficult.
The present invention has been completed in view of this situation and has an object of providing a method for drying a honeycomb formed body using microwaves, by which drying of the honeycomb formed bodies without causing cell deformation by sufficiently reducing the difference in the drying speed inside the honeycomb formed bodies during the drying process is possible.